Pressure Variation Assisted Fiber Extraction and Development of High Performance Natural Fiber Composites and Nanocomposites
Date of Award
Doctor of Philosophy
It is believed, that due to the large surface areas provided by the nano scale materials, various composite properties could be enhanced when such particles are incorporated into a polymer matrix. There is also a trend of utilizing natural resources or reusing and recycling materials that are already available for the fabrication of the new composite materials. Cellulose is the most abundant natural polymer on the planet, and therefore it is not surprising to be of interest for composite fabrication. Basic structures of cellulose, comprised of long polysaccharide chains, are the building blocks of cellulose nano fibers. Nano fibers are further bound into micro fibrils and macro fibers. Theoretically pure cellulose nano fibers have tremendous strengths, and therefore are some of the most sought after nano particles. The fiber extraction however is a complex task. The ultrasound, which creates pressure variation in the medium, was employed to extract nano-size cellulose particles from microcrystalline cellulose (MCC). The length and the intensity of the cavitations were evaluated. Electron microscopy studies revealed that cellulose nanoparticles were successfully obtained from the MCC after ultrasound treatment of just 30 minutes. Structure of the fractionated cellulose was also analyzed with the help of X-ray diffraction, and its thermal properties were evaluated with the help of differential scanning calorimetry (DSC). Ultrasound treatment performed on the wheat straw, kenaf, and miscanthus particles altered fiber structure as a result of the cavitation. The micro fibers were generated from these materials after they were subjected to NaOH treatment followed by the ultrasound processing. The potential of larger than nano-sized natural fibers to be used for composite fabrication was also explored. The agricultural byproducts, such as wheat or rice straw, as well as other fast growing crops as miscanthus or kenaf, are comprised of three basic polymers. Just like in wood the polymers are: cellulose, hemicelluloses, and lignin. When subjected to elevated pressures and temperatures, we are able to get access to some of these natural polymers and use them as a matrix material for composite fabrication. Therefore, fabrication of composite materials without addition of synthetic polymers is possible. Thermal and mechanical properties of such composites are evaluated with the help of electron microscopy, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and flexural strength measurements. The chemical changes in the composites are also probed with the help of Fourier transform infrared (FTIR) analysis. Various additives introduced into composite materials provide different properties. The addition of small amounts of synthetic polymers further enhances the properties of natural fiber composites and do not require high fabrication pressures. Calcium sulfite crystals, which are one of the coal combustion products, were combined with the natural fibers and recycled HDPE polymer to form wood substitute composites. The introduction of these additives resulted in composites with the properties similar to those of the natural wood. Coal combustion products, often used in composite material fabrication, contain mercury which may be rereleased during composite fabrication. Mercury behavior under composite fabrication conditions, such as elevated pressures and temperatures were evaluated. Sulfite rich scrubber material, generated during the flue gas desulphurization process was the main target of the study. It was observed that the release of the mercury is highly dependent on the composite fabrication pressure as well as the temperature.
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